In 1959 a semiconductor-to-metal phase transition temperature for vanadium dioxide (VO2) of 340 K was reported. Below 340 K, VO2 is a semiconductor with monoclinic unit cell symmetry. At temperatures above 340 K, VO2 becomes metallic and adopts tetragonal unit cell symmetry. With this phase transition VO2 experiences many drastic changes, such as a rapid decrease in optical transmittance in the near-IR and an increase in resistivity of ca. five orders of magnitude. These properties allow the use of VO2 in a wide range of applications, including thermochromic materials, electrical switches, optical storage, self-protecting support windows, erasable optical data recording, thermal sensors, coatings for energy-efficient windows and thermal sensors and relays.
Indeed, VO2 is one of several transition metal oxides which show an abrupt change in certain physical properties such as electrical resistance at a temperature Tt (transition temperature). In VO2, the transition is probably best described as a first-order semiconductor-to-metal transition accompanied by a lattice distortion with Tt=68° C. Because of this conveniently low transition temperature and the large drop in near infrared transmittance, films of VO2 have been used in a variety of applications as noted above.
Substances like VO2 in which both sensing and actuating capabilities are coupled by an intrinsic control mechanism are sometimes referred to as “smart” materials. The above-described phase transition of VO2 is accompanied by extraordinary changes in its electronic and optical properties. When a VO2 film is coated onto a transparent substrate, and illuminated so that the film absorbs sufficiently intense laser light, the resulting temperature increase can induce a rapid semi-conducting-to-metal phase transformation. The presence of the metallic VO2 phase then produces a reflecting surface, that subsequently strongly attenuates further transmission of the incident laser radiation through the coated substrate. Accordingly, the VO2 film performs both sensing and actuating functions through coupled intrinsic properties of the material.
For investigation of the semiconductor-to-metal phase transition, VO2 has been prepared as thin films. The following methods have been reported for the preparation of thin films of VO2; sol-gel processing of V(V) precursors [Livage, J. Optical and Electrical Properties of Vanadium Oxides Synthesized from Alkoxides. Coord. Chern. Rev. 1999, 190-192, 391-403; Speck, K. R.; Hu, H. S.-W.; Sherwin, M. E.; Potember, R. S. Vanadium Dioxide Films Grown from Vanadium Tetraisopropoxide by the Sol-Gel Process. Thin Solid Films 1988, 165, 317-322; Dachuan, Y.; Niankan, X.; Jingyu, Z.; Xiulin, Z. Vanadium Dioxide Films with Good Electrical Switching Properties. J. Phys. D: Appl. Phys. 1996, 29, 1051-1057; Livage, J.; Guzman, G.; Beteille, F.; Davidson, P. Optical Properties of Sol-Gel Derived vanadium Oxide Films. J. Sol-Gel Sci. Technol. 1997, 8, 857-865; Partlow, D. P.; Gurkovich, S. R.; Radford, K. C.; Denes, L. J. Switchable Vanadium Oxide Films by a Sol-Gel Process. J. Appl. Phys. 1991, 70 (1), 443-452]; chemical vapor deposition of VO(OiPr)3 [Sahana, M. B.; Subbanna, G. N.; Shivashankar, S. A. Phase Transformation and Semiconductor-Metal Transition in Thin Films of VO2 Deposited by Low-pressure Metalorganic Chemical Vapor Deposition. J. Appl. Phys. 2002, 92 (11), 6495-6504; Greenberg, C. B. Undoped and Doped VO2 Films Grown from VO(OC3H7)3 Thin Solid Films 1983, 110, 73-82; Golubev, V. G.; Davydov, V. Y.; Kartenko, N. F.; Kurdyukov, D. A.; Medvedev, A. V.; Pevtsov, A. B.; Scherbakov, A. V.; Shadrin, E. B. Phase Transition-Governed Opal-VO2 Photonic Crystal. Appl. Phys. Lett. 2001, 79 (14), 2127-2129]; radio-frequency sputtering using vanadium metal, V203, and V205 as targets [Wang, X.; Xu, J.; Fei, Y.; Li, D.; Li, T.; Nie, Y.; Feng, K.; Wu, N. Preparation of Thermochromic VO2 Thin Films on Fused Silica and Soda lime Glass by R F Magnetron Sputtering. Jpn. J. Appl. Phys. 2002, 41, 312-313.; Shigesato, Y.; Enomoto, M.; Odaka, H. Thermochromic VO2 Films Deposited by RF Magnetron Sputtering Using V203 or V20S Targets. Jpn. J. Appl. Phys. 2000, 39, 6016-6024. Hanlon, T. J.; Walker, R. E.; Coath, J. A.; Richardson, M. A. Comparison Between Vanadium Dioxide Coating on Glass Produced by Sputtering, Alkoxide, and Aqueous Sol-Gel Methods. Thin Solid Films 2002, 405, 234237]; ion-beam enhanced deposition (IBED) from V205 powder [Ninhyi, Y.; Jinhua, L.; Chan, H. L. W.; Chenglu, L. Comparison of VO2 Thin Films Prepared by Inorganic Sol-Gel and IBED Methods. Appl. Phys. A 2004, 78, 777-780; Li, J.; Yuan, N. Temperature Sensitivity of Resistance of VO2 Polycrystalline Films Formed by Modified Ion Beam Enhanced Deposition. Appl. Surf. Sci. 2004, 233, 252-257]; pulsed laser deposition (PLD) using a vanadium metal Target [(Suh, J. Y.; Lopez, R; Feldman, L. C.; Haglund, R F., Jr. Semiconductor to Metal Phase Transition in the Nucleation and Growth of VO2 Nanoparticles and Thin Films. J. Appl. Phys. 2004, 96 (2), 1209-1213; Lopez, R; Feldman, L. C.; Haglund, R F., Jr. Size-Dependent Optical Properties of VO2 Nanoparticle Arrays. Phys. Rev. Lett. 2004, 93 (17), 177403-1/177403-4; Soltani, M.; Chaker, M.; Haddad, E.; Kruzelecky, R. V.; Nikanpour, D. Optical Switching of Vanadium Dioxide Films Deposited by Reactive Pulsed Laser Deposition. J. Vac. Sci. Technol. A 2004, 22 (3), 859-864; Liu, H.; Vasquez, O.; Santiago, V. R.; Diaz, L.; Fernandez, F. E. Excited State Dynamics and Semiconductor-to-Metallic Phase Transition of VO2 Thin Film. J. Lumin. 2004, 108, 233-238] and ion implantation of vanadium and oxygen ions into a Si02 substrate [Lopez, R; Haynes, T. E.; Boatner, L. A.; Feldman, L. C.; Haglund, R. F., Jr. Size Effects in the Structural Phase Transitions of VO2 Nanoparticles. Phys. Rev. B 2002, 65, 224113; Lopez, R.; Haynes, T. E.; Boatner, L. A.; Feldman, L. C.; Haglund, R. F., Jr. Temperature-Controlled Surface Plasmon Resonance in VO2 Nanorods. Opt. Lett. 2002, 27(15), 1327-1329; Lopez, R.; Boatner, L. A.; Haynes, T. E.; Haglund, R. F., Jr.; Feldman, L. C. Enhanced hysteresis in the semiconductor-to-metal phase transition of VO2 precipitates formed in Si02 by ion implantation. Appl. Phys. Lett. 2001, 79 (19), 3161-3163; Lopez, R.; Boatner, L. A.; Haynes, T. E.; Feldman, L. C.; Haglund, R. F., Jr. Synthesis and characterization of size-controlled vanadium dioxide nanocrystals in a fused silica matrix. J. Appl. Phys. 2002, 92 (7), 40314036; Lopez, R.; Boatner, L. A.; Haynes, T. E.; Haglund, R. F., Jr.; Feldman, L. C. Switchable Reflectivity on Silicon from a Composite VO2-Si02 Protecting Layer. Appl. Phys. Lett. 2004, 85 (8), 1410-1412; Lopez, R.; Suh, J. Y.; Feldman, L. C.; Haglund, R. F., Jr. Ion Beam Lithographic Fabrication of Ordered VO2 Nanoparticle Arrays. Mater. Res. Soc. Syrnp. Proc. 2004, 820, 319-324].
Since vanadium (IV) oxide is a metastable oxide between the very stable vanadium (V) and (III) oxides, thermal treatment of as-prepared samples is important for the preparation of vanadium (IV) oxide (VO2). The following thermal treatments are reported for the preparation of VO2 thin films; (1) heating of V(V) sol-gel precursor films at 400-700° C. in N2(g) (via thermal elimination of O2), supra; at 500° C. in 5% H2/95% Ar(g) [Soltani, M.; Chaker, M.; Haddad, E.; Kruzelecky, R. V.; Nikanpour, D. Optical Switching of Vanadium Dioxide Films Deposited by Reactive Pulsed Laser Deposition. J. Vac. Sci. Technol. A 2004, 22 (3), 859-864; Liu, H.; Vasquez, O.; Santiago, V. R.; Diaz, L.; Fernandez, F. E. Excited State Dynamics and Semiconductor-to-Metallic Phase Transition of VO2 Thin Film. J. Lumin. 2004, 108, 233-238]; at 300-500° C. in H2(g) [Dachuan, U.; Niankan, X.; Jingyu, Z.; Xiulin, Z. High Quality Vanadium Dioxide Films Prepared by an Inorganic Sol-Gel Method. Mater. Res. Bull. 1996, 31 (3), 335-340] and at 500° C. and 5 Pa in a vacuum oven, supra (2) reactive decomposition of CVD precursors at 500-550° C. in N2(g) flow [Greenberg, C. B. Undoped and Doped VO2 Films Grown from VO(OCSH7h Thin Solid Films 1983, 110, 73-82]; (3) heating of IBED V205 samples in Ar(g) at 500-600° C., [supra and Lee, M.-H.; Cho, J.-S. Better Thermochromic Glazing of Windows with Anti-Reflection Coating. Thin Solid Films 2000, 365, 5-6] and (4) heating VxOylSi PLD precursors at 450° C. and 250 mTorr O2 pressure to give VO2/Si, supra.
Micelles and inverse micelles are microscopic vesicles that contain amphipathic molecules but do not contain an aqueous volume that is entirely enclosed by a membrane. In micelles the hydrophilic part of the amphipathic compound is on the outside (on the surface of the vesicle) whereas in inverse micelles the hydrophobic part of the amphipathic compound is on the outside. The inverse micelles thus contain a polar core that can solubilize both water and macromolecules within the inverse micelle. As the volume of the core aqueous pool increases the aqueous environment begins to match the physical and chemical characteristics of bulk water. See U.S. Pat. No. 6,673,612. The resulting inverse micelle can be referred to as a microemulsion of water in oil (Schelly, Z. A. Current Opinion in Colloid and Interface Science, 37-41, 1997; Castro, M. J. M., Cabral, J. M. S. Biotech. Adv. 6, 151-167, 1988).
Sol-gel processing can be used to prepare a variety of materials including unsupported powders, fibers, xerogel and aerogel composites, and dense glasses or ceramics. Sol-gel processing consists of a combination of hydrolysis and polycondensation reactions of a metal alkoxide, M(OR)x, resulting in the formation of a sol (a suspension of colloidal particulate materials in a liquid media) and a gel (a porous three-dimensional solidified framework surrounded by a sustaining liquid phase). In sol-gel processing, the particle size and morphology of the product material are determined by catalyst concentration, water concentration, and relative rates of hydrolysis and condensation. The type of catalyst, acidic or basic, also affects the properties of the product formed.
The use of an acid catalyst causes products to elongate in a chain-like fashion forming a porous gel. The formation of a branching metal center occurs with the use of a base catalyst leading to the formation of a more dense gel. The size of the R group of the metal alkoxide reactant also affects the rates of hydrolysis and condensation. A larger R group sterically shields a metal center and reduces the rates of hydrolysis and condensation reactions. Silicon alkoxides are the most common alkoxides used in sol-gel processing because they have convenient rates of hydrolysis and condensation. Transition metal alkoxides tend to undergo hydrolysis and condensation at extremely rapid rates due to high electrophilicity and possible coordination number expansion of the metal center. R-group exchange of transition metal alkoxides is frequently performed to decrease their reaction rates. Transition metal methoxides and ethoxides are frequently reacted with acetylacetonate (acac) to form the corresponding M(acac)x complex with decreased rates of hydrolysis and condensation. Often R groups are exchanged in parent alcohols with larger R groups than the alkoxide to reduce the reaction rates of the alkoxide.
Inverse micelle reactions are commonly used for the shape-directed synthesis of nanoparticles [Landfester, K. Recent Developments in Miniemulsions—Formation and Stability Mechanisms. Macromol. Symp. 2000, 150, 171-178; Chang, C.-L.; Fogler, H. S. Controlled Formation of Silica Particles from Tetraethyl Orthosilicate in Nonionic Water-in-Oil Microemulsions. Langmuir 1997, 13, 3295-3307; Landfester, K. The Generation of Nanoparticles in Miniemulsions. Adv. Mater. 2001, 13 (10), 765-768; Arriagada, F. J.; Osseo-Asare, K. Controlled hydrolysis of tetraethyoxysilane in a nonionic water-in-oil microemulsion: a statistical model of silica nucleation. Colloids Surf. A 1999, 154, 311-326; Arriagada, F. J.; Osseo-Asare, K. Phase and dispersion stability effects in the synthesis of silica nanoparticles in a non-ionic reverse microemulsion. Colloids Surf. A 1992, 69, 105-115; Arriagada, F. J.; Osseo-Asare, K. Synthesis of Nanosize Silica in a Nonionic Water-in-Oil Microemulsion: Effects of the Water/Surfactant Molar Ratio and Ammonia Concentration. J. Colloid Interface Sci. 1999, 211, 210-220].
An inverse micelle reaction medium generally consists of an oil or organic phase, a surfactant system, and aqueous reagents. Cyclohexane and the like may be used as the oil/organic phase, and hexanol may be used as a co-surfactant. Triton N-101 and Igepal® CO-630 are commonly used as surfactants. The latter are polyoxyethylenenonylphenyl ether reagents that form spherical inverse micelles that act as vesicles in which the aqueous reagents react. To initiate the formation of these vesicles, aqueous ammonia is often used as a base catalyst. This inverse micelle system allows for the formation of nanoparticulate powders with a narrow size distribution. The size of the product powders is determined by controlling the size of the inverse micelles formed. As the water-to-surfactant molar ratio increases, the micelle size decreases, giving smaller particles and narrower size distribution of the resulting particles.
Polydispersity and particle agglomeration are the most common problems associated with uncontrolled hydrolysis and condensation. With inverse micelle systems, reactions are confined to small aqueous vesicles allowing for better control of hydrolysis and condensation reaction rates. Therefore, by choosing the appropriate sol-gel reagents and inverse micelle water to surfactant ratio, the formation of monodispersed particles is available from the combination of sol-gel processing and inverse micelle synthesis methods.
A variety of methods have been reported for the preparation of nanocrystals. These methods include inverse micelle preparations, arrested precipitation, aerosol processes, and sol-gel processes (see U.S. Pat. No. 6,682,596).
The recent discovery of a particle size-dependence on the phase transition temperature of VO2 for particles of 70-180 nm average diameter has been reported. Nanoparticulate VO2/Si(Si02) samples for these studies were prepared using two different syntheses strategies; (1) ion implantation of vanadium and oxygen ions in a Si02 substrate with subsequent annealing at 1000° C. in flowing high-purity Ar(g) and (2) pulsed laser deposition of V on a Si substrate using a vanadium metal target at 5 mTorr O2 pressure and subsequent thermal treatment at 450° C. and 250 mTorr O2 pressure. Other methods for the production of VO2 are disclosed in U.S. Pat. Nos. 5,885,665; 5,608,568; 4,957,725; 4,654,231.
It is an object of the invention to provide novel vanadium dioxide nanoparticles and methods for the preparation thereof.